perf sample: Make user_regs and intr_regs optional

The struct dump_regs contains 512 bytes of cache_regs, meaning the two
values in perf_sample contribute 1088 bytes of its total 1384 bytes
size. Initializing this much memory has a cost reported by Tavian
Barnes <tavianator@tavianator.com> as about 2.5% when running `perf
script --itrace=i0`:
https://lore.kernel.org/lkml/d841b97b3ad2ca8bcab07e4293375fb7c32dfce7.1736618095.git.tavianator@tavianator.com/

Adrian Hunter <adrian.hunter@intel.com> replied that the zero
initialization was necessary and couldn't simply be removed.

This patch aims to strike a middle ground of still zeroing the
perf_sample, but removing 79% of its size by make user_regs and
intr_regs optional pointers to zalloc-ed memory. To support the
allocation accessors are created for user_regs and intr_regs. To
support correct cleanup perf_sample__init and perf_sample__exit
functions are created and added throughout the code base.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250113194345.1537821-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

1 files changed, 20 insertions, 9 deletions

diff --git a/tools/perf/util/scripting-engines/trace-event-python.c b/tools/perf/util/scripting-engines/trace-event-python.c
index b1b5e94537e4..520729e78965 100644
--- a/tools/perf/util/scripting-engines/trace-event-python.c
+++ b/tools/perf/util/scripting-engines/trace-event-python.c
@@ -745,19 +745,30 @@ static int set_regs_in_dict(PyObject *dict,
 	const char *arch = perf_env__arch(evsel__env(evsel));
 
 	int size = (__sw_hweight64(attr->sample_regs_intr) * MAX_REG_SIZE) + 1;
-	char *bf = malloc(size);
-	if (!bf)
-		return -1;
+	char *bf = NULL;
 
-	regs_map(&sample->intr_regs, attr->sample_regs_intr, arch, bf, size);
+	if (sample->intr_regs) {
+		bf = malloc(size);
+		if (!bf)
+			return -1;
 
-	pydict_set_item_string_decref(dict, "iregs",
-			_PyUnicode_FromString(bf));
+		regs_map(sample->intr_regs, attr->sample_regs_intr, arch, bf, size);
 
-	regs_map(&sample->user_regs, attr->sample_regs_user, arch, bf, size);
+		pydict_set_item_string_decref(dict, "iregs",
+					_PyUnicode_FromString(bf));
+	}
 
-	pydict_set_item_string_decref(dict, "uregs",
-			_PyUnicode_FromString(bf));
+	if (sample->user_regs) {
+		if (!bf) {
+			bf = malloc(size);
+			if (!bf)
+				return -1;
+		}
+		regs_map(sample->user_regs, attr->sample_regs_user, arch, bf, size);
+
+		pydict_set_item_string_decref(dict, "uregs",
+					_PyUnicode_FromString(bf));
+	}
 	free(bf);
 
 	return 0;